Conventionally, as disclosed in Patent Document 1, in an exhaust-gas flow path of an internal combustion engine, an exhaust manifold, a catalytic converter, a sub muffler, a main muffler, and a tail pipe are disposed in the exhaust-gas flow path and installed in a space under a floor of a vehicle.
In the space under the floor of the vehicle, not only devices of exhaust system, but also many components of the vehicle, such as a propeller shaft, a cross member, a side member, a fuel tank, and others, are provided. For this reason, since the space under the floor is limited, a length of each of exhaust pipes to be connected to a muffler is determined in accordance with an arrangement of the muffler.
Exhaust noise generally includes explosion noise inside a cylinder(s) of an internal combustion engine, gas-flow noise generated by a flow of exhaust gases, a standing wave generated by air-column resonance due to lengths of exhaust pipes, and so on. In this regard, depending on lengths of exhaust pipes, silencing performance with respect to a low-frequency noise may be deteriorated because of an influence of the standing wave generated within each of these exhaust pipes. The low-frequency noise causes a muffled sound inside a vehicle, and this muffled sound is a factor of giving an uncomfortable feeling to a passenger in the vehicle. In order to suppress the muffled sound, a main muffler with a large capacity or a sub muffler is used.
For example, as shown in FIG. 4A, in a case where a length of an exhaust pipe 102 connected to a downstream side of a main muffler 100 is long and therefore, a standing wave caused by air-column resonance of harmful low-frequency waves is generated in the exhaust pipe 102, a resonant frequency is increased to a frequency which does not pose a problem by increasing the capacity of the main muffler 100 or disposing a sub muffler 104 somewhere within the length of the exhaust pipe 102 to shorten an actual length of the exhaust pipe 102, as shown in FIG. 4B.
When an amount of noise reduction is increased by increasing the capacity of the main muffler 100 or additionally providing the sub muffler 104, peaks in sound pressure level due to the standing wave decrease and the overall sound pressure level also decreases, as shown by the solid line in FIG. 4C. The broken line in FIG. 4C shows a relationship between a number of rotation of an internal combustion engine and sound pressure level, in a case where a sub muffler is not disposed. In FIG. 4C, a vertical axis shows the sound pressure level, and a horizontal axis shows the number of rotation of the internal combustion engine.